Pathogen virulence is a main focus in evolutionary biology. Virulence theory investigates when harmful strains can spread, and is also useful to predict the evolution of beneficial symbionts. This theory predicts that beneficial bacteria which undergo infectious transmission can evolve to parasitize their hosts. Rhizobial symbionts of plants offer an intriguing case. Rhizobia are nitrogen-fixing bacteria critical to the growth of many legumes. These bacteria form symbiotic nodules on legume roots and fix nitrogen, a main resource for the plants. However, agricultural evidence suggests that rhizobia can parasitize legumes, by consuming plant carbon but then fixing no nitrogen. Here, we propose a combined field and lab approach to investigate rhizobial parasitism: I) We will ascertain the prevalence of rhizobial parasites and their effects on the fitness of wild legumes, II) We will study the evolution of rhizobial parasitism: phylogenetic analyses of sequenced isolates will reveal whether rhizobial parasitism arises sporadically or if parasites represent diverged clades and will distinguish among molecular models for the evolution of parasitism. [unreadable] [unreadable] [unreadable]